The present invention relates to a catalyst for effectively removing nitrogen oxides (NOx) by using ammonia (NH3) as a reducing agent. Particularly, the present invention relates to a Ti-PILC (titania-pillared interlayer clay) catalyst impregnated by vanadia, which is manufactured by the generally known technology. More specifically, the present invention relates to a vanadia catalyst impregnated onto a Ti-PILC, in which a NOx contained in the exhaust system from an electric power plant and the like (an excessive amount of SO2 included in the flue gas) is reacted with NH3 (which is injected as a reducing agent) over a vanadia catalyst impregnated onto a Ti-PILC, so that they can be converted into nitrogen and water.
Many countries have been making efforts for the progress of their own industries, and in this process, they are commonly encountered with a serious environmental pollution and contamination. Particularly, since 1980""s, the global warming problem, the damage of the ozone layer and the acid rain are adding the seriousness to the environmental pollution and contamination. In this context, the importance of keeping the environmental safety is drawing an attention. NOx is chemical compound which causes the formation of the acid rain and the photochemical smog. Therefore, NOx is considered as the main source for the environmental pollution and contamination. Therefore, the industrialized nations such as the United States, Germany, Japan and others are strengthening the regulations on the emissions of NOx, and the regulations also become to be reinforced in Korea.
NOx occurs from the electric power plants, internal combustion engines and the manufacturing process of nitric acid. They include seven compounds such as nitrogen monoxide (NO), nitrogen dioxide (NO2), nitrogen trioxide (NO3), nitrous oxide (N2O), dinitrogen trioxide (N2O3), dinitrogen tetraoxide (N2O4), and dinitrogen pentaoxide (N2O5). However, those which are contained in the atmospheric air are mainly N2O, NO and NO2. Generally, NOx indicates NO and NO2, but the most of NOx which is emitted from electric power plants and automobiles are NO. Therefore, it is important to develop a technology for lowering the release of this compound.
Recently, the variety of technology for removing NOx from electric power plants have been developed, and among them, the selective catalytic reduction (SCR) method is the one which is satisfactory in the technical, economical and legal aspects. In this technology, NOx is removed from the exhaustion gas by using a catalyst, and therefore, the dependence of the catalyst on the total cost of the process is significant. The wide spectrum of the catalysts has been suggested from metal oxide catalysts to zeolite catalysts. The catalyst can be easily deactivated due to the sulfur compounds commonly contained flue gas. However, vanadia-titania catalyst has been generally known as the sulfur tolerance SCR catalyst. Since the catalyst is employed for treating the large amount of flue gas which is discharged from electric power plants and the like, it should be superior in the mechanical strength and in the formability into the low pressure drop reactor such as honeycomb. The titania fixed catalyst contains strong sulfur tolerance and high NO removal activity. However, there are drawbacks for the catalyst such as a high cost, a low specific surface area, and a weak mechanical strength. Therefore, the SCR catalyst should exhibit high NO removal activity with strong physico-chemical properties, and furthermore, it should be tolerable to the poisoning of the catalyst caused by SO2, the alkaline metals such as sodium (Na), calcium (Ca), and potassium (K), and arsenic trioxide (As2O3).
Since 1970""s, there have been intensive researches to develop the SCR catalyst for the removal of NO by NH3. In U.S. Pat. No. 4,048,112 (dated 1976) by Matsushita et al., a catalyst is disclosed in which vanadia is impregnated onto titania. Since that time, many researches have been carried out on the vanadia-titania catalysts. Further, in U.S. Pat. No. 4,085,193 (dated 1978) by Nakajima et al., various metal compound catalysts are disclosed which employs titania as the catalyst support, and includes various metals such as Mo, W, Fe, V, Ni, Co, Cu, Cr, U and the like. Recently, in order to improve the disadvantages of present SCR catalysts, the various physicochemical functions were included into the new generation of the SCR catalyst. Particularly, the sulfur tolerance of the catalyst for SO2 commonly contained in the flue gas as well as NO is very important for the commercialization of the SCR process. As described before, the disadvantages of the titania as the carrier for the SCR process are attempted to be improved. For this purpose, many researches have been carried out to develop a new carrier which is related to the properties of the titania.
In U.S. Pat. No. 4,735,927 (dated 1988) and U.S. Pat. No. 4,735,930 (dated 1988) by Gerdes et al., the catalysts for removing NOx are disclosed which contains anatase titania including a proper amount of hydrogen type or ammonium type zeolite and a proper amount of sulfate by impregnating a vanadium, molybdenum or copper compound onto the support.
In U.S. Pat. No. 4,929,586 (dated 1990) by Hegedus et al., a titania containing the bimodal pore size distribution at about 60 nm is employed as the carrier for SCR catalyst, thereby improving NOx removal activity and the catalyst deactivation. The titania is mixed with the porous inorganic oxides such as SiO2, Al2O3, ZrO2, AlPO4, Fe2O3 and their mixtures. V2O5, MoO3, WO3, Fe2O3, CuSO4, VOSO4, SnO2, Mn2O3, and Mn3O4 are employed as an active component on the catalyst surface.
In U.S. Pat. No. 5,415,850 (dated 1995) by Yang et al., a selective reducing method using various PILC catalysts is disclosed. Here, in the PILC catalysts, iron oxide, chromia, titania, zirconia and alumina are pillared, and among them, the most superior one is the delaminated Fe-PILC. Furthermore, when chromia and iron oxide are impregnated onto the Ti-PILC at a proper ratio, the SCR reaction for reducing NOx by NH3 as a reducing agent showed the most superior performance. These catalysts also contain the bimodal pore size distributions depending on the preparation method of PILC, and the catalyst deactivation has been improved. However, vanadia has never been suggested as an active component for PILC catalyst.
The present invention is intended to overcome the above-described disadvantages of the conventional SCR catalyst.
Therefore, it is an object of the present invention to provide a new catalyst in which the disadvantages of the conventional vanadia-titania catalysts are complemented, so that a superior activity for removing NOx can be realized, and that the catalyst exhibit strong tolerance against the various deactivation factors.
It is another object of the present invention to provide a catalyst containing a superior mechanical strength, a high surface area and a versatile pore size distribution. It will achieve high performance of NO removal activity compared to the conventional vanadia-titania catalyst. The catalyst deactivation can also be improved. In addition, extrudability of the catalyst for the development of a low pressure drop reactor such as honeycomb can be improved. For Ti-PILC, titania is pillared into space between the layers of the clay.
It is still another object of the present invention to provide a catalyst in which the operation temperature window shifts to the lower reaction temperature so as to improve the energy economy, and NOx removal activity is improved so as to decrease the size of the reactor enhancing the pressure drop over the reactor, thereby achieving an economical and efficient operation of the SCR process.